Ultrasonic wave is propagated through material and reflected on an interface between the material and a foreign substance or a vacancy contained in the material, because such foreign substance and vacancy shows an acoustic impedance differing from that of the material. Utilizing this characteristic, ultrasonic wave can be employed for detecting foreign substances and defects in the target material without destruction of the target material. Accordingly, non-destructive inspection using ultrasonic wave is used in a variety of technical fields such as diagnostics, inspections of metallic material, inspections of buildings, and detections of a group of fishes in the seas or lakes.
The ultrasonic wave can be produced by applying a voltage of a high frequency to a piezoelectric transducer which is manufactured by placing an electrode on each surface side of a piezoelectric sheet and then applying a voltage between these electrodes. The piezoelectric transducer receives a reflected ultrasonic wave and convert the received ultrasonic wave to electric signals. Various materials showing the piezoelectric characteristic are known. For example, the piezoelectric sheet can be formed of a single crystal such as crystallized quartz, a sintered piezoelectric ceramic, a piezoelectric polymer material, and a mixture of a piezoelectric ceramic and a polymer resin. A piezoelectric sheet of sintered piezoelectric ceramic is widely employed for manufacture of a piezoelectric transducer because it gives a high electromechanical coupling factor. Examples of the piezoelectric ceramics include barium titanate, lead titanate, and lead zirconate titanate (Pb(Zr, Ti)O.sub.3), which is generally referred to as PZT or P-PZT (i.e., piezoelectric PZT). PZT is most widely utilized.
The piezoelectric sheet to be employed as a source of ultrasonic wave in a ultrasonic detector preferably shows an acoustic impedance similar to that of the material to be inspected. Further, it is preferred for the piezoelectric sheet to emit a ultrasonic wave having a mono pulse wave form so that the defects and foreign substances in the inspected material are accurately detected.
In the fields of ultrasonic inspection, there are fields in which the ultrasonic wave should be propagated into water, air, or living body having water and air therein. For instance, ultrasonic diagnostic system, ultrasonic flow meter system, and ultrasonic blood flow meter system are mentioned. It is a problem that the acoustic impedance of water, air, and living body is lower than the acoustic impedance of ordinary piezoelectric ceramic materials.
In order to adjust the difference of acoustic impedance between the piezoelectric ceramic sheet and the inspected material and to enhance sensitivity of ultrasonic inspection equipment, an acoustic impedance-adjusting material is conventionally attached to a piezoelectric transducer of the inspection equipment. However, the adjustment of acoustic impedance using the acoustic impedance-adjusting material is limitative.
Japanese Journal of Applied Physics, Vol. 30, No. 9B, September, 1991, pp. 2271-2273 describes a porous piezoelectric PZT ceramic transducer exhibiting improved characteristics such as a large piezoelectric voltage coefficient (g constant), a low acoustic impedance (Z.sub.0) and a low Q. In the publication, a sintered PZT disk having a diameter of 50 mm and a thickness of 8 mm is prepared by mixing powders of PZT and PMMA (poly(methyl methacrylate)) having average particle sizes of 1.2 .mu.m and 30 .mu.m, pressing the mixture to form a disc, heating the disc to 400.degree. C. to burn out PMMA and finally sintering the heated disc.
Japanese Journal of Applied Physics, Vol. 31, Part 1, No. 9B, September, 1992, pp. 3067-3069 describes transmitting properties of porous PZT transducer which is produced in the same manner as that described in the above-identified publication.
Japanese Patent Provisional Publication No. 1-172281 describes a porous dielectric material for piezoelectric transducer having pore volume of 23 to 45% and main pores having a size of 20 to 44 .mu.m. No mention is given with respect to the preparation of a thin piezoelectric sheet.
Japanese Patent Provisional Publication No. 1-171281 describes a porous piezoelectric transducer having a coat of glass material on each surface side and an electrode on each glass material coat.
Japanese Patent Provisional Publication No. 4-300253 describes a process for producing a porous piezoelectric element using a PZT powder, a binder, and a combustible polymer powder. No mention is given with respect to the preparation of a thin piezoelectric sheet.
In the detection of small defective portions and small foreign substances in the target material, a ultrasonic wave having a frequency of 1 to 30 MHz is employed. In order to emit a ultrasonic wave of a frequency of 1-30 MHz, a piezoelectric transducer should have a piezoelectric sheet of 0.05 to 2.0 mm thick. Moreover, the piezoelectric transducer having such a small thickness still has a uniform piezoelectric characteristic within the piezoelectric sheet.
Until now, there has been not known a thin porous piezoelectric ceramic sheet which is capable of reliably emitting a ultrasonic wave having a frequency of 1 to 30 MHz and further capable of receiving a reflected ultrasonic wave at a satisfactorily high sensitivity.